1. Percentage Composition

2. Percent Part X 100% Whole Percent by mass = Mass of element X 100%
Mass of compound

3. Percent Composition Can be determined by analysis.
Or, if you know the formula:
Just assume you have 1 mole
Divide the mass of each element by the formula mass of the compound

4. Percent Composition of H2O
2 X H = 2 X 1.0 = 2.0
1 X O = 1 X 16.0 = 16.0
Molar mass = = 18.0
%H = (2.0/18.0) X 100% = 11.2%
%O = (16.0/18.0) X 100% = 88.8%

5. Percent Composition of C6H12O6
6 X C = 6 X 12.0 = 72.0
12 X H = 12 X 1.0 = 12.0
6 X O = 6 X 16.0 = 96.0
Molar mass = grams
% C =
% H =
% O =
(72.0/180.0) X 100% = 40.0%
(12.0/180.0) X 100% = 6.67%
(96.0/180.0) X 100% = 53.3%

6. Percent Composition of NaCl
Molar mass of NaCl = 58.5 g/mole
% Na = 23.0/58.5 X 100% = 39.3%
% Cl = 35.5/58.5 X 100% = 60.7%
(Or, 100% % = 60.7%)

7. Percentage Composition
They won’t remind you that you have to calculate the formula mass!